This invention relates generally to ink jet printers. More specifically, the invention relates to a system for providing liquid ink at proper static pressure to a series of drop-on-demand ink jet print orifices.
As is known, ink jet printing systems utilize printing heads in which ink droplets are emitted through one or more orifices and onto a target surface. In impulse type drop-on-demand printing, the emission of ink through the orifice is controlled by creating pressure pulses within an ink chamber in the printhead. Referring to prior art FIG. 1, a typical printhead 2 is provided with an array of orifices 3, which are individually controlled to emit ink droplets 4 that form the desired image on the target surface as the surface moves relative to the printing head. The orifices are fed ink from individual chambers S within each printhead, which communicate with corresponding input ports 6. Ink is supplied via capillary action to each input port 6 from a reservoir 7 through ink supply lines 9.
It is important that the proper static pressure, typically a small negative static pressure, is achieved at each ink jet orifice to avoid orifice drool. Static pressure within the printhead is largely a function of the static fluid pressure at the printhead input port, since capillary forces within a given printhead offset any appreciable variation in pressure head among the orifices in that printhead. Thus, the static pressure at the input port influences the pressure at the ink jet orifices which are in immediate proximity to the input port. The optimal static pressure is determined by the physical properties of the ink, such as viscosity and surface tension, wetability and the substrate material used to construct the orifices. Ordinarily, the optimal static pressure of the ink is negative one to three inches of water. As can be seen in FIG. 1, and as is known from hydraulic theory, the static pressure at the input port is a function of the difference in height (H) between the input port and the reservoir ink level. More particularly, the static pressure at each orifice is a function of the difference in height between each orifice and the reservoir ink level.
In commercial ink jet printing applications, it is advantageous to provide a printing apparatus having a large printing area to permit imaging of a large image on a target surface without multiple passes of the surface past the printing head. This may be accomplished by providing a number of printheads vertically stacked or "stitched" together. Vertically stacked printhead arrangements, however, present special problems associated with the control of static pressure, and their commercial advantages have heretofore been limited by increasing costs or complexity relating to ink storage and delivery. It is not feasible to supply all printheads in a stacked arrangement from a common reservoir because uniform static pressure cannot be achieved. As seen in FIG. 1, printheads disposed above the reservoir fluid level experience negative static pressure, while those disposed below the reservoir experience positive static pressure. Moreover, while it is feasible to provide each printhead with a separate, level-controlled reservoir as illustrated in FIG. 2, the cost of such an arrangement is prohibitive. Additionally, the space limitations and tight spacing between printheads make it physically impractical to install reservoirs and level-control devices in this configuration.
Prior art distribution systems are not readily adaptable to stacked printhead arrangements because of the complexity and costs associated with manufacturing and maintenance. There is thus desired an ink jet fluid distribution system which may be easily and inexpensively constructed and which provides proper and dependable control of the static pressure of ink delivered to each printhead in a stacked arrangement.